Is it required to have a cycle in an animation?
When you open an action and Blender displays a frame, Blender looks for the bones associated with the keyed channels on that frame. Only channels with fcurves of the action are updated, unkeyed channels remain with the same values.

If a character is posed sitting down and a nod action is opened (which only has keys for the head), only these (head) keys are going to update the bones (the head). It doesn't matter in which position the characters other bones were, when the nod action was created, only the bones are remembered which are keyed. For the other bones, there is nothing to evaluate.

A single key  is enough to create a horizontally extrapolated fcurve. The solution to your request is to simply key all bones on at least on frame of the animation. If this clutters up your action editor with too many channels, you can always regroup them and/or change their order by selecting them and pressing &#8670; Page up/&#8671; Page down. Most actions will look like the image below. If you want bones to preserve a certain position, key them all!